Hall Effect devices are used in sensor applications for contactless sensing of magnetic fields. Hall Effect sensors are capable of being implemented on semiconductor chips using CMOS technology. This has resulted in Hall Effect sensors being one of the most widely used types of magnetic sensors. Standard CMOS Hall devices, however, are typically only capable of detecting magnetic fields that are perpendicular to the surface of the semiconductor chip.
Various methods have been developed to detect magnetic fields that are parallel to the surface of a semiconductor chip. One method uses magnetic concentrators to “steer” magnetic fields toward the surface of the chip so that they can be detected using standard Hall Effect sensors. The magnetic concentrators, however, add cost to and increase the complexity of manufacturing the device.
Another method that has been developed for detecting magnetic fields parallel to the chip surface is the use of so-called vertical Hall Effect devices. Vertical Hall Effect devices include a plurality of contacts in a well arranged in a straight line parallel to the surface of the chip. Two of the contacts are connected to a bias current or voltage to introduce a current path through the chip. The output voltage is used to measure the output voltage of the Hall device. In the presence of a magnetic field, the carriers that are moving along the path are deflected by a Lorentz force and a Hall electric field is formed.
One difficulty faced in the use of vertical Hall Effect sensors, however, is the voltage offset that is typically introduced into the output voltage of the sensor due to various factors, such as fabrication imperfections and environmental conditions. The presence of the voltage offset in the sensor output compromises the accuracy of the magnetic field measurement obtained by the Hall device.
To compensate for voltage offset in Hall Effect devices, a spinning current technique has been developed for the sensor in which the contacts of the Hall device that are used to connect the bias source and the contacts used to connect provide the output voltage are switched to provide a plurality of bias modes. Each bias mode can provide a slightly different output voltage. As a result, the output voltage is modulated which enables the offset voltage, which appears as a DC component of the signal, to be identified and compensated for in the measurement of the magnetic field.
The spinning current method requires a highly symmetric device to be effective. Vertical Hall Effect devices, however, are geometrically asymmetric due to the linear arrangement of the contacts. Therefore, one of the most difficult tasks in the development of vertical Hall Effect sensors is determining an arrangement or connection configuration for the contacts of a vertical Hall device that enables a symmetric response so that the spinning current method can be used to cancel the offset.
One example of a previously known vertical Hall Effect device includes a linear arrangement of four contacts. The four-contact vertical Hall Effect device is capable of providing a high degree of symmetry between bias modes which allows the spinning current technique to be used to cancel the offset. However, the four contact device exhibits a large pre-spinning systematic offset which, besides presenting a significant dynamic range challenge to the readout electronics, results in the feed through of the noise in the Hall sensor bias circuitry to the final output, which degrades the signal to noise ratio.
To reduce the pre-spinning offset voltage, vertical Hall devices have been provided with five or even six contacts in a linear arrangement. The five-contact vertical Hall Effect device is highly asymmetric between bias modes which results in the ineffectiveness of the spinning current technique to cancel offset.
In a previously known six-contact vertical Hall Effect device, the outer contacts are shorted together and the inner contacts are used as a four-contact Hall device. The six-contact sensor has a high degree of symmetry between it bias modes, and can be adapted to exhibit a small pre-spinning systematic offset. However, the spacing required to achieve a nominally zero offset and maximum sensitivity for the sensor require deeper wells to allow adequate spacing between contacts. As a result, the sensor size may be too big or too inconvenient to implement in some devices.